The invention concerns a pressing cushion for use in laminating presses, having a cushioning that is elastic in the thickness direction and having connecting threads, going back and forth between the surfaces of the pressing cushion, that are configured at least partially as thermally conductive threads whose thermal conductivity is better than that of the cushioning.
The manufacture of layered materials, for example decoratively coated particle boards, is performed in laminating presses that can be configured as low- or high-pressure multiplaten presses or short-cycle presses. To ensure that the pressure of the press plates is transferred uniformly onto the pressed material over its entire surface, pressing cushions are inserted between the pressed material and the press plates. The pressing cushions must be capable of withstanding high pressures as well as the temperatures that occur in such presses, and they must be capable of transferring the heat proceeding from the press plates quickly and without major losses onto the pressed material. It is also desirable to be able to process pressed materials in different formats in succession with one pressing cushion.
The known pressing cushions can be divided into two groups. The one group involves layered designs. For example, DE-A-26 27 442 describes a pressing cushion in which a matted nonwoven fabric is coated on both sides with a silicone elastomer. EP-A-0 235 582 discloses a pressing cushion in which a silicone elastomer film is coated on both sides with a parting film. U.S. Pat. No. 3,723,3320 discloses a pressing cushion made of a silicone film that can additionally be equipped with a parting film.
Pressing cushions that are made of or contain a textile thread system in the form of a fabric are known in many embodiments, especially as regards the materials used therein. DE-B-23 19 593 discloses a pressing cushion whose basis is a metal sieve fabric that is enclosed in a matrix made of a silicone elastomer. The pressing cushion according to DE-A-23 38 749 has a glass fiber fabric in which individual or all threads are impregnated or coated with a plastic, for example a silicone elastomer, so that these threads form the cushioning. The teaching of DE-A-26 50 642 is similar. EP-A-0 493 630 proposes a pressing cushion made of a textile fabric in which the fabric is composed of aromatic polyamide threads and metal threads as thermally conductive threads. DE-U-295 18 204 discloses a pressing cushion in which one portion of the threads has a silicone elastomer and a further portion can be configured as metal wire. DE-U-94 18 984.6 describes a pressing cushion in which the threads can be made of a wide variety of materials, in particular of rubber, silicone elastomer, or metal, and combinations thereof. Lastly, DE-U-297 21 495 and DE-U-297 21 494 depict pressing cushions made of a fabric in which the individual threads are interwoven and configured in a particular fashion.
With the first group of pressing cushions, satisfactory thermal conductivity cannot be achieved. With the second group of pressing cushions, compromises must be made between thermal conductivity and dimensional stability, so that an optimum result in terms of those properties cannot always be obtained.
It is therefore the object of the invention to configure a pressing cushion in such a way that the thermal conductivity and dimensional stability can be adjusted within wide limits, and that in terms of the cushioning as well, a wide range of possible variations exists for designing the pressing cushion in accordance with requirements.
According to the present invention, this object is achieved in that connecting threads are provided which have a sewn profile or form knitted stitches. The basic idea of the invention is thus for at least a portion of the connecting threads, preferably all of the connecting threads, not to be interwoven with the cushioning, but rather for the connection to be made by sewing (which is also to be understood as quilting or embroidering) or knitting. It has been found that a strong overall structure having high dimensional stability can be achieved thereby. It is moreover possible, by way of the proportion of thermally conductive threads among the connecting threadsxe2x80x94and the connecting threads can also all be configured as thermally conductive threadsxe2x80x94and by way of the nature and quantity of the stitches, to adapt the thermal conductivity to the particular requirements within wide limits. A greater freedom in terms of configuring the cushioning also results. In this context, xe2x80x9cthermally conductive threadsxe2x80x9d are to be understood as those threads which, because of their presence, improve the thermal conductivity of the pressing cushion in the thickness direction. If further connecting threads are present in addition to the stitch-forming connecting threads, these threads can be configured entirely or partially as thermally conductive threads, either in combination with thermally conductive threads of the stitch-forming connecting threads or indeed alone, if the stitch-forming connecting threads do not contain thermally conductive threads in the aforesaid sense. The overall result is to make available a structure for the pressing cushion that offers a substantially greater breadth of variation in terms of the aforesaid properties than has been attainable with the previously known fabric configurations.
The cushioning can be configured as a single-ply cushioning layer. There also exists the possibility, however, of configuring the cushioning with multiple plies and configuring at least one of the plies elastically in the thickness direction, all of the plies being interconnected by connecting threads.
As a detail, the elastic ply or at least one of the elastic plies can have or be made of cushion threads that are elastic in the thickness direction.
As an embodiment of this basic idea, the cushion threads can extend next to one another in the same direction in the particular ply; the cushion threads can be present as a layer or can be part of a fabric or of a knitted material. In this context, multiple plies having cushion threads can also be arranged one on top of another. It is advantageous in this case if the cushion threads of the plies extend in different directions, for example diagonally, transversely, or longitudinally, so that they cross one another.
The cushion threads can be made, for example, of an elastomeric material, such as silicone elastomer or fluorosilicone elastomer. They can, however, also be structured, in a manner known per se, in such a way that they each have a core thread made, for example, of metal wire that is surrounded by an elastomeric thread material, for example silicone elastomer. In addition, the cushion threads can also be at least partially surrounded by metal wire, for example in overbraided, wound, or stranded form, as is evident for example from DE-U-295 18 204 and DE-U-297 21 495. The elastomeric thread sheath can also, for example, be constituted from elastomeric threads that are wound or stranded around the core thread.
Instead of or in combination with cushion threads, the elastic ply or at least one of the elastic plies can also be made of an elastomer film. The film can be equipped with the elements that improve thermal conductivity, for example powders, threads, or fibers made of metal such as aluminum, bronze, stainless steel, copper, or brass. For reinforcement, the elastomer film can also contain a textile support that can be present as a fabric, thread layer, or knitted fabric.
In addition, the elastic ply or at least one of the elastic plies can also be configured as an elastic woven fabric or knitted fabric, or as a fiber batt, or as a combination of the aforesaid textiles.
The elastic ply or at least one of the elastic plies can, however, also be made of foamed material; the foamed material can contain the elements that improve thermal conductivity. Another possibility consists in providing a nonwoven ply; the nonwoven ply can have metal fibers in order to improve thermal conductivity. It is also possible, however, instead or in combination, to provide a fabric, thread layer, or a knitted material, the respective ply being made partially or entirely of metal threads. Combinations of the aforesaid plies are also possible, for example a combination of fabric, thread layer, or knitted material with a nonwoven ply.
If the pressing cushion is of multiple-layer configuration, it may also be advantageous if at least one of the plies is of incompressible configuration, being made for example as a metal foil or of foil strips.
As regards the thermally conductive threads, metals with good thermal conductivity, for example aluminum, bronze, stainless steel, copper, or brass, are suitable. The thermally conductive threads can, as wires or strands, be made entirely of these metals or can continuously contain them. It is possible in this context to configure the thermally conductive threads as plastic threads, for example from high-strength plastics such as aramid or polyimide, with a metal content.
Provision is also made, according to the invention, for thermally conductive threads and thermally non-conductive threads to alternate as weave threads [sic], the thermally non-conductive threads having a greater tensile strength than the thermally conductive threads, for example being made of aramid or polyimide.
Lastly, the invention provides for the pressing cushion to have edge thickenings, which can be achieved by the use of additional threads or ribbons, or forming loops at the edge. Folding the edges over and then sewing them up also yields such edge thickenings. This results in an equalization of pressure all the way into the edge regions.
The invention is illustrated in more detail, with reference to exemplary embodiments, in the drawings, in which: